RAM shortage?

When I bought my PC, the advertising blurb said 512mb RAM, when I look in System properties, it says memory; 348mb RAM. Have I been short-changed on memory, or is the rest of the RAM in use? Although no programmes were running at the time?

I have just looked on the PCWorld site, In 512 RAM they have 2 - DDR, 2 - DDR DIMM and 2 - DDR2 listed, what is the difference? or to simplify that, which one do I need? I assume Corsair to be the make of my existing, does the 400 have any signifigance?